FIG. 1 shows an exemplary network structure of an Evolved Universal Mobile Telecommunications System (E-UMTS) as a mobile communication system to which a related art and the present invention are applied. The E-UMTS system is a system that has evolved from the existing UMTS system, and its standardization work is currently being performed by the 3GPP standards organization. The E-UMTS system can also be referred to as a LTE (Long-Term Evolution) system.
The E-UMTS network can roughly be divided into an E-UTRAN and a Core Network (CN). The E-UTRAN generally comprises a terminal (i.e., User Equipment (UE)), a base station (i.e., eNode B), an Access Gateway (AG) that is located at an end of the E-UMTS network and connects with one or more external networks. The AG may be divided into a part for processing user traffic and a part for handling control traffic. Here, an AG for processing new user traffic and an AG for processing control traffic can be communicated with each other by using a new interface. One eNode B may have one or more cells. An interface for transmitting the user traffic or the control traffic may be used among the eNode Bs. The CN may comprise an AG, nodes for user registration of other UEs, and the like. An interface may be used to distinguish the E-UTRAN and the CN from each other.
The various layers of the radio interface protocol between the mobile terminal and the network may be divided into a layer 1 (L1), a layer 2 (L2) and a layer 3 (L3), based upon the lower three layers of the Open System Interconnection (OSI) standard model that is well-known in the field of communications systems. Among these layers, Layer 1 (L1), namely, the physical layer, provides an information transfer service to an upper layer by using a physical channel, while a Radio Resource Control (RRC) layer located in the lowermost portion of the Layer 3 (L3) performs the function of controlling radio resources between the terminal and the network. To do so, the RRC layer exchanges RRC messages between the terminal and the network. The RRC layer may be located by being distributed in network nodes such as the eNode B, the AG, and the like, or may be located only in the eNode B or the AG.
FIG. 2 shows exemplary control plane architecture of a radio interface protocol between a terminal and a UTRAN (UMTS Terrestrial Radio Access Network) according to the 3GPP radio access network standard. The radio interface protocol as shown in FIG. 2 is horizontally comprised of a physical layer, a data link layer, and a network layer, and vertically comprised of a user plane for transmitting user data and a control plane for transferring control signaling. The protocol layer in FIG. 2 may be divided into L1 (Layer 1), L2 (Layer 2), and L3 (Layer 3) based upon the lower three layers of the Open System Interconnection (OSI) standards model that is widely known in the field of communication systems.
Hereinafter, particular layers of the radio protocol control plane of FIG. 2 and of the radio protocol user plane of FIG. 3 will be described below.
The physical layer (Layer 1) uses a physical channel to provide an information transfer service to a higher layer. The physical layer is connected with a medium access control (MAC) layer located thereabove via a transport channel, and data is transferred between the physical layer and the MAC layer via the transport channel. Also, between respectively different physical layers, namely, between the respective physical layers of the transmitting side (transmitter) and the receiving side (receiver), data is transferred via a physical channel.
The Medium Access Control (MAC) layer of Layer 2 provides services to a radio link control (RLC) layer (which is a higher layer) via a logical channel. The RLC layer of Layer 2 supports the transmission of data with reliability. It should be noted that if the RLC functions are implemented in and performed by the MAC layer, the RLC layer itself may not need to exist. The PDCP layer of Layer 2 performs a header compression function that reduces unnecessary control information such that data being transmitted by employing Internet Protocol (IP) packets, such as IPv4 or IPv6, can be efficiently sent over a radio interface that has a relatively small bandwidth.
The Radio Resource Control (RRC) layer located at the lowermost portion of Layer 3 is only defined in the control plane, and handles the control of logical channels, transport channels, and physical channels with respect to the configuration, reconfiguration and release of radio bearers (RB). Here, the RB refers to a service that is provided by Layer 2 for data transfer between the mobile terminal and the UTRAN.
As for channels used in downlink transmission for transmitting data from the network to the mobile terminal, there is a Broadcast Channel (BCH) used for transmitting system information, and a downlink Shared Channel (SCH) used for transmitting user traffic or control messages. A downlink multicast, traffic of broadcast service or control messages may be transmitted via the downlink SCH or via a separate downlink Multicast Channel (MCH). As for channels used in uplink transmission for transmitting data from the mobile terminal to the network, there is a Random Access Channel (RACH) used for transmitting an initial control message, and an uplink Shared Channel (SCH) used for transmitting user traffic or control messages.
As for downlink physical channels for transmitting information transferred via the channels used in downlink transmission over a radio interface between the network and the terminal, there is a Physical Broadcast Channel (PBCH) for transmitting BCH information, a Physical Multicast Channel (PMCH) for transmitting MCH information, a Physical Downlink Shared Channel (PDSCH) for transmitting PCH and a downlink SCH information, and a Physical Downlink Control Channel (PDCCH) (also, referred to as ‘DL L1/L2 control channel’) for transmitting control information provided by the first and second layers such as a DL/UL Scheduling Grant, and the like. As for uplink physical channels for transmitting information transferred via the channels used in uplink transmission over a radio interface between the network and the terminal, there is a Physical Uplink Shared Channel (PUSCH) for transmitting uplink SCH information, a Physical Random Access Channel (PRACH) for transmitting RACH information, and a Physical Uplink Control Channel (PUCCH) for transmitting control information provided by the first and second layers, such as a HARQ ACK or NACK, a Scheduling Request (SR), a Channel Quality Indicator (CQI) report, and the like.
Description of an uplink timing alignment maintenance in a LTE system will be given. In the LTE system that based on an Orthogonal Frequency Division Multiplex (OFDM) technology, there is possibility of interferences between terminals (UEs) during a communication between UE and base station (eNB). In order to minimize interferences between terminals, it is important that the base station must manage or handle a transmission timing of the UE. More particularly, the terminal may exist in random area within a cell, and this implies that a data transmission time (i.e., traveling time of data from UE to base station) can be varied based on a location of the terminal. Namely, if the terminal is camped on edge of the cell, data transmission time of this specific terminal will be much longer than data transmission time of those terminals who camped on a center of the cell. In contrast, if the terminal is camped on the center of the cell, data transmission time of this specific terminal will be much shorter than data transmission time of those terminals who camped on the edge of the cell. The base station (eNB) must manage or handle all data or signals, which are transmitted by the terminals within the cell, in order to prevent the interferences between the terminals. Namely, the base station must adjust or manage a transmission timing of the terminals upon each terminal's condition, and such adjustment can be called as the timing alignment maintenance. One of the methods for maintaining the timing alignment is a random access procedure. Namely, during the random access procedure, the base station receives a random access preamble transmitted from the terminal, and the base station can calculate a time alignment (Sync) value using the received random access preamble, where the time alignment value is to adjust (i.e., faster or slower) a data transmission timing of the terminal. The calculated time alignment value can be notified to the terminal by a random access response, and the terminal can update the data transmission timing based on the calculated time alignment value. In other method, the base station may receive a sounding reference symbol (SRS) transmitted from the terminal periodically or randomly, the base station may calculate the time alignment (Sync) value based on the SRS, and the terminal may update the data transmission timing according to the calculated time alignment value.
As explained above, the base station (eNB) may measure a transmission timing of the terminal though a random access preamble or SRS, and may notify an adjustable timing value to the terminal. Here, the time alignment (Sync) value (i.e., the adjustable timing value) can be called as a time advance command (referred as ‘TAC’ hereafter). The TAC may be process in a MAC (Medium Access control) layer. Since the terminal does not camps on a fixed location, the transmission timing is frequently changed based on a terminal's moving location and/or a terminal's moving velocity. Concerning with this, if the terminal receives the time advance command (TAC) from the base station, the terminal expect that the time advance command is only valid for certain time duration. A time alignment timer (TAT) is used for indicating or representing the certain time duration. As such, the time alignment timer (TAT) is started when the terminal receives the TAC (time advance command) from the base station. The TAT value is transmitted to the terminal (UE) through a RRC (Radio Resource Control) signal such as system information (SI) or a radio bearer reconfiguration. Also, if the terminal receives a new TAC from the base station during an operation of the TAT, the TAT is restarted. Further, the terminal does not transmit any other uplink data or control signal (e.g., data on physical uplink shared channel (PUSCH), control signal on Physical uplink control channel (PUCCH) except for the random access preamble when the TAT is expired or not running.
As explained above, in LTE system, a base station (eNB) transmits a timing advance command (TAC) to a terminal (UE) for maintenance of an uplink timing alignment, the terminal applies the received TAC, and the time alignment timer (TAT) starts upon receiving the TAC. A detail description of the TAT value will be given. When the terminal access to a particular cell, the terminal receives the TAT value through system information (SI) and starts the TAT through a random access procedure. The TAT value is a common value used in a cell. If the base station wants to set the terminal with a certain TAT value, the base station transmits the certain TAT value included in a radio bearer reconfiguration message. The above certain TAT value is a dedicated value that assigned to a corresponding terminal. The reason that the base station set the assigned TAT value to the terminal is because the terminal can not possibly perform an uplink data transmission even if the time Sync of the terminal is aligned in following conditions; 1) if particular terminal does not move in a particular cell, 2) if the TAT is expired even if the time alignment of the terminal is not changed, etc. Therefore, the base station has to provide a new TAC to the terminal before an expiration of the TAT. Here, in order to prevent a waste of radio resource for transmitting an unnecessary TAC transmission, for those terminal do not move frequently may set with a high TAT value so as to minimize number of TAC transmission.
However, if the base station wants to set a particular terminal with a specifically assigned TAT value while the particular terminal is in active state and the particular terminal use a common TAT received via system information, in related art, the base station must perform an Intra-cell-Handover inn order to change a current TAT value to the specifically assigned TAT value. However, since the Intra-cell-Handover contains a random access procedure, a reconfiguring the TAT by Intra-Cell-Handover may cause great drawbacks of collision errors, a delayed time due to a collision, or massive radio resource(s) usage, etc.